Digital amplifiers using switching amplifiers (class-D amplifiers) have been used as audio amplifiers that drive speakers or the like, owing to, for example, the high power efficiency of the digital amplifiers. The digital amplifiers are classified as a type to which analog signals are input or a type to which digital signals are input; digital amplifiers of the latter type are called fully digital amplifiers. The fully digital amplifier can generate output signals without the need for analog input signals. The fully digital amplifier thus has advantages of enabling cost reduction of an audio system and improving performance of the audio system while maintaining high energy efficiency.
Operation of a common fully digital amplifier will be described below. An example of a configuration of the fully digital amplifier is shown in FIG. 2. A sound source signal r[i] is a pulse code modulated (PCM) signal. If the sound source signal is obtained from, for example, a CD, the sound source signal r[i] has a sampling frequency of 44.1 kHz. The sound source signal r[i] is input to an over sampler 4, which then converts the sound source signal r[i] into a PCM signal u[k] of 705.6 kHz, which is 16 times as high as the sampling frequency of the sound source signal r[i]. The PCM signal u[k] is then converted, by a quantizer 1, into a PCM signal y[k] having the same sampling period but coarsely quantized. The resolution of the PCM signal y[k] is determined by the quantizer 1 and is the same as that of a pulse width modulator 2. The PCM signal y[k] is converted into a pulse width modulated (PWM) signal w(t) by the pulse width modulator 2. A noise-shaping filter 3, compensates for quantization noise generated by the quantizer 1 and signal distortion generated by the pulse width modulator 2 by feedback, so that an audible frequency component of the PWM signal w(t) corresponds to an audible frequency component of the PCM signal u[k]. Thus, the audible frequency component of the PWM signal w(t) corresponds to the sound source signal r[i]. The PWM signal w(t) is supplied to a switching amplifier 5, which then converts the PWM signal w(t) into a power signal. The power signal then passes through a low-pass filter 6 composed of L and C, and is supplied to a speaker that is a load.
Patent Document 1 discloses a low-distortion pulse width modulation signal generator which allows the signal distortion resulting from the pulse width modulation to be fed back to the noise-shaping filter to reduce harmonic distortion, the low-distortion pulse width modulation signal generator superposing a feed forward signal for compensation of the harmonic distortion on an input to the quantizer.    Patent Document 1: Japanese Patent Laid-Open No. 2004-236617